Digital i/q imbalance compensation in a quadrature receiver

ABSTRACT

An apparatus and method reduce distortion in a processed signal. The apparatus includes a first receive path, a second receive path, a summation unit, and a compensation unit. The first receive path is configured to process a received analog signal into a first digital signal. The second receive path is configured to process the received analog signal with a phase shift into a second digital signal. The summation unit is configured to sum the first and second digital signals to form a processed digital signal. The compensation unit is configured to identify a conjugate of the processed digital signal, apply a weighting factor to the conjugate of the processed digital signal to form a weighted signal, and subtract the weighted signal from the processed digital signal to reduce the distortion.

CROSS-REFERENCE TO RELATED APPLICATION(S) AND CLAIM OF PRIORITY

The present application is related to U.S. Provisional PatentApplication No. 61/408,419, filed Oct. 29, 2010, entitled “DIGITALPROGRAMMABLE ADAPTIVE WIDEBAND I/Q IMBALANCE COMPENSATION IN QUADRATURERECEIVERS”. Provisional Patent Application No. 61/408,419 is assigned tothe assignee of the present application and is hereby incorporated byreference into the present application as if fully set forth herein. Thepresent application hereby claims priority under 35 U.S.C. §119(e) toU.S. Provisional Patent Application No. 61/408,419.

TECHNICAL FIELD OF THE INVENTION

The present application relates generally to receivers in a wirelessnetwork and, more specifically, to digital I/Q imbalance compensation ina quadrature receiver.

BACKGROUND OF THE INVENTION

Quadrature modulation and demodulation schemes allow two message signalsto be conveyed on a single carrier wave. The two message signals areusually out of phase or phase shifted by about ninety degrees. In aquadrature receiver, the two message signals can be retrieved from areceived signal. For example, the quadrature receiver may includeseparate receiver paths, one of the receiver paths applying a phaseshift to the received signal. The resulting signals are generallyreferred to as the ‘I’ (real) and the ‘Q’ (phase shifted) signals.

The I/Q signals are processed using different components along theseparate receiver paths. Differences in the components used asprocessing applied to the I/Q signals can create an imbalance in betweenprocessed I/Q signals. For example, components along the I/Q paths mayhave different gain or frequency parameters. Additionally, due tohardware tolerances the phase shift applied to the Q signal may not beexactly ninety degrees. The differences along the I/Q paths createsself-imposed interference or distortion in the resulting signal. Thedistortion in the resulting signal raises the noise floor in basebandsignal processing, which may result in signal loss and/or increasedprocessing complexity.

Different I/Q imbalance compensation schemes attempt to reduce thisself-imposed interference or distortion in different ways. However,current I/Q imbalance compensation schemes suffer from one or more ofthe following problems: 1) narrowband, 2) static, fixed compensation, 3)data/tone aided training, 4) not programmable, and/or 5) analog.

Therefore, there is a need for an improved I/Q imbalance compensationscheme. In particular, there is a need for a quadrature receiver that iscapable of digital programmable adaptive wideband I/Q imbalancecompensation.

SUMMARY OF THE INVENTION

In one illustrative embodiment, an apparatus reduces distortion in aprocessed signal. The apparatus includes a first receive path, a secondreceive path, a summation unit, and a compensation unit. The firstreceive path is configured to process a received analog signal into afirst digital signal. The second receive path is configured to processthe received analog signal with a phase shift into a second digitalsignal. The summation unit is configured to sum the first and seconddigital signals to form a processed digital signal. The compensationunit is configured to identify a conjugate of the processed digitalsignal, apply a weighting factor to the conjugate of the processeddigital signal to form a weighted signal, and subtract the weightedsignal from the processed digital signal to reduce the distortion.

In another illustrative embodiments, a receiver for reducing distortionin a processed signal is provided. The receiver includes a pair ofquadrature receiver paths, a conjugate unit, an adaptation module, aweighting module, and a decoder. The pair of quadrature receiver pathsis configured to process a received analog signal into a digital signal.The conjugate unit is configured to identify a conjugate of theprocessed digital signal. The adaptation module includes a filter bank.The adaptation module is configured to identify a set of weightingfactors for one or more frequencies of the conjugate of the processeddigital signal based on frequency dependent distortion identified usingthe filter bank. The weighting module is configured to apply the set ofweighting factors to the conjugate of the processed digital signal tofrom a weighted signal and subtract the weighted signal from theprocessed digital signal to reduce the distortion. The decoder isconfigured to decode the processed digital signal.

In yet another illustrative embodiment, a method for reducing distortionin a processed signal in a receiver in a wireless communications networkis provided. The method includes receiving an analog signal, processingthe analog signal into a digital signal using a pair of quadraturereceiver paths to form a processed digital signal, identifying aconjugate of the processed digital signal, applying a weighting factorto the conjugate of the processed digital signal to form a weightedsignal, and subtracting the weighted signal from the processed digitalsignal to reduce the distortion.

Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, itmay be advantageous to set forth definitions of certain words andphrases used throughout this patent document: the terms “include” and“comprise,” as well as derivatives thereof, mean inclusion withoutlimitation; the term “or,” is inclusive, meaning and/or; the phrases“associated with” and “associated therewith,” as well as derivativesthereof, may mean to include, be included within, interconnect with,contain, be contained within, connect to or with, couple to or with, becommunicable with, cooperate with, interleave, juxtapose, be proximateto, be bound to or with, have, have a property of, or the like; the term“set of” with reference to items means one or more items; and the term“controller” means any device, system or part thereof that controls atleast one operation, such a device may be implemented in hardware,firmware or software, or some combination of at least two of the same.It should be noted that the functionality associated with any particularcontroller may be centralized or distributed, whether locally orremotely. Definitions for certain words and phrases are providedthroughout this patent document, those of ordinary skill in the artshould understand that in many, if not most instances, such definitionsapply to prior, as well as future uses of such defined words andphrases.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and itsadvantages, reference is now made to the following description taken inconjunction with the accompanying drawings, in which like referencenumerals represent like parts:

FIG. 1 illustrates an exemplary wireless system which transmits messagesaccording to an illustrative embodiment of the present disclosure;

FIG. 2 illustrates a high-level diagram of an orthogonal frequencydivision multiple access transmit path according to an illustrativeembodiment of the present disclosure;

FIG. 3 illustrates a high-level diagram of an orthogonal frequencydivision multiple access receive path according to an illustrativeembodiment of the present disclosure;

FIG. 4 illustrates a quadrature receiver with I/Q imbalance compensationin accordance with an illustrative embodiment of the present disclosure;

FIG. 5 illustrates a quadrature receiver with I/Q imbalance compensationincluding an adaptation module according to one embodiment of thepresent disclosure;

FIG. 6 illustrates a filter bank for a quadrature receiver according toan illustrative embodiment of the present disclosure;

FIG. 7 illustrates a quadrature receiver with I/Q imbalance compensationincluding an adaptation module according to another embodiment of thepresent disclosure; and

FIG. 8 illustrates a process for reducing distortion in a processedsignal according to an illustrative embodiment of the presentdisclosure.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 through 8, discussed below, and the various embodiments used todescribe the principles of the present disclosure in this patentdocument are by way of illustration only and should not be construed inany way to limit the scope of the disclosure. Those skilled in the artwill understand that the principles of the present disclosure may beimplemented in any suitably arranged wireless communication system.

FIG. 1 illustrates an exemplary wireless system, which transmitsmessages according to an illustrative embodiment of the presentdisclosure. In the illustrated embodiment, wireless system 100 includesbase station (BS) 101, base station (BS) 102, base station (BS) 103, andother similar base stations (not shown). Base station 101 is incommunication with base station 102 and base station 103. Base station101 is also in communication with Internet 130 or a similar IP-basedsystem (not shown).

Base station 102 provides wireless broadband access (via base station101) to Internet 130 to a first plurality of subscriber stations withincoverage area 120 of base station 102. The first plurality of subscriberstations includes subscriber station 111, which may be located in asmall business (SB), subscriber station 112, which may be located in anenterprise (E), subscriber station 113, which may be located in a WiFihot spot (HS), subscriber station 114, which may be located in a firstresidence (R), subscriber station 115, which may be located in a secondresidence (R), and subscriber station 116, which may be a mobile device(M), such as a cell phone, a wireless laptop, a wireless PDA, or thelike.

Base station 103 provides wireless broadband access (via base station101) to Internet 130 to a second plurality of subscriber stations withincoverage area 125 of base station 103. The second plurality ofsubscriber stations includes subscriber station 115 and subscriberstation 116. In an exemplary embodiment, base stations 101-103 maycommunicate with each other and with subscriber stations 111-116 usingOFDM or OFDMA techniques.

While only six subscriber stations are depicted in FIG. 1, it isunderstood that wireless system 100 may provide wireless broadbandaccess to additional subscriber stations. It is noted that subscriberstation 115 and subscriber station 116 are located on the edges of bothcoverage area 120 and coverage area 125. Subscriber station 115 andsubscriber station 116 each communicate with both base station 102 andbase station 103 and may be said to be operating in handoff mode, asknown to those of skill in the art.

Subscriber stations 111-116 may access voice, data, video, videoconferencing, and/or other broadband services via Internet 130. In anexemplary embodiment, one or more of subscriber stations 111-116 may beassociated with an access point (AP) of a WiFi WLAN. Subscriber station116 may be any of a number of mobile devices, including awireless-enabled laptop computer, personal data assistant, notebook,handheld device, or other wireless-enabled device. Subscriber stations114 and 115 may be, for example, a wireless-enabled personal computer(PC), a laptop computer, a gateway, or another device.

FIG. 2 is a high-level diagram of an orthogonal frequency divisionmultiple access (OFDMA) transmit path. FIG. 3 is a high-level diagram ofan orthogonal frequency division multiple access (OFDMA) receive path.In FIGS. 2 and 3, the OFDMA transmit path is implemented in base station(BS) 102 and the OFDMA receive path is implemented in subscriber station(e.g. subscriber station 116 of FIG. 1), and the OFDMA receive path 300may be implemented in a base station (e.g. base station 102 of FIG. 1)for the purposes of illustration and explanation only.

Transmit path 200 comprises channel coding and modulation block 205,serial-to-parallel (S-to-P) block 210, Size N Inverse Fast FourierTransform (IFFT) block 215, parallel-to-serial (P-to-S) block 220, addcyclic prefix block 225, up-converter (UC) 230. Receive path 300comprises down-converter (DC) 255, remove cyclic prefix block 260,serial-to-parallel (S-to-P) block 265, Size N Fast Fourier Transform(FFT) block 270, parallel-to-serial (P-to-S) block 275, channel decodingand demodulation block 280.

At least some of the components in FIGS. 2 and 3 may be implemented insoftware while other components may be implemented by configurablehardware or a mixture of software and configurable hardware. Inparticular, it is noted that the FFT blocks and the IFFT blocksdescribed in this disclosure document may be implemented as configurablesoftware algorithms, where the value of Size N may be modified accordingto the implementation.

Furthermore, although this disclosure is directed to an embodiment thatimplements the Fast Fourier Transform and the Inverse Fast FourierTransform, this is by way of illustration only and should not beconstrued to limit the scope of the disclosure. It will be appreciatedthat in an alternate embodiment of the disclosure, the Fast FourierTransform functions and the Inverse Fast Fourier Transform functions mayeasily be replaced by Discrete Fourier Transform (DFT) functions andInverse Discrete Fourier Transform (IDFT) functions, respectively. Itwill be appreciated that for DFT and IDFT functions, the value of the Nvariable may be any integer number (i.e., 1, 2, 3, 4, etc.), while forFFT and IFFT functions, the value of the N variable may be any integernumber that is a power of two (i.e., 1, 2, 4, 8, 16, etc.).

In transmit path 200, channel coding and modulation block 205 receives aset of information bits, applies coding (e.g., LDPC coding) andmodulates (e.g., Quadrature Phase Shift Keying (QPSK) or QuadratureAmplitude Modulation (QAM)) the input bits to produce a sequence offrequency-domain modulation symbols. Serial-to-parallel block 210converts (i.e., de-multiplexes) the serial modulated symbols to paralleldata to produce N parallel symbol streams where N is the IFFT/FFT sizeused in BS 102 and SS 116. Size N IFFT block 215 then performs an IFFToperation on the N parallel symbol streams to produce time-domain outputsignals. Parallel-to-serial block 220 converts (i.e., multiplexes) theparallel time-domain output symbols from Size N IFFT block 215 toproduce a serial time-domain signal. Add cyclic prefix block 225 theninserts a cyclic prefix to the time-domain signal. Finally, up-converter230 modulates (i.e., up-converts) the output of add cyclic prefix block225 to RF frequency for transmission via a wireless channel. The signalmay also be filtered at baseband before conversion to RF frequency.

The transmitted RF signal arrives at SS 116 after passing through thewireless channel and reverse operations to those at BS 102 areperformed. Down-converter 255 down-converts the received signal tobaseband frequency and remove cyclic prefix block 260 removes the cyclicprefix to produce the serial time-domain baseband signal.Serial-to-parallel block 265 converts the time-domain baseband signal toparallel time domain signals. Size N FFT block 270 then performs an FFTalgorithm to produce N parallel frequency-domain signals.Parallel-to-serial block 275 converts the parallel frequency-domainsignals to a sequence of modulated data symbols. Channel decoding anddemodulation block 280 demodulates and then decodes the modulatedsymbols to recover the original input data stream.

Each of base stations 101-103 may implement a transmit path that isanalogous to transmitting in the downlink to subscriber stations 111-116and may implement a receive path that is analogous to receiving in theuplink from subscriber stations 111-116. Similarly, each one ofsubscriber stations 111-116 may implement a transmit path correspondingto the architecture for transmitting in the uplink to base stations101-103 and may implement a receive path corresponding to thearchitecture for receiving in the downlink from base stations 101-103.

FIG. 4 illustrates a quadrature receiver with I/Q imbalance compensationin accordance with an illustrative embodiment of the present disclosure.Signals received by receiver 400 are passed onto paths 402 and 404.

In this illustrative example, local oscillator 406 in combination withphase shifter 408 and mixer 410 phase shift the signal along path 404 byabout ninety degrees. Mixers 410 and 412 modify I/Q signals on paths 402and 404 to an intermediate frequency. Amplifiers 414 and 416 amplify apower of I/Q signals on paths 402 and 404. Analog-to-digital converters418 and 420 convert I/Q analog signals on paths 402 and 404 digital I/Qsignals. Filters 422 and 424 filter out unwanted signals on paths 402and 404. The resulting processed digital signals are the summed bysummation unit 426 to form digital signal 428. Paths 402 and 404 areillustrative examples of receiver paths. Other hardware configurationsmay be used in addition to or instead of the components depicted onpaths 402 and 404.

The different illustrative embodiments of the present disclosurerecognize and take into account that any one of the components alongpaths 402 and 404 may cause an imbalance between the I and Q signalssummed at summation unit 426. For example, phase shifter 408 may have atolerance causing the phase shift applied at mixer 410 to be greater orless than ninety degrees, resulting in a phase imbalance between the Iand Q signals. In another example, the amplitude of the I and Q signalsmay be modified differently due to phase shifter 408 being applied tojust path 404 or to operational differences between amplifiers 414 and416 and/or analog-to-digital converters 418 and 420. These are examplesof frequency independent imbalances. In some examples, the gain ofamplifiers 414 and 416 may not be flat. Thus, signals at differentfrequencies may be amplified differently. This example is an example offrequency dependent imbalance.

The different illustrative embodiments of the present disclosurerecognize and take into account that attempting to account for alldifferences in components along paths 402 and 404 may be difficult andmay vary for each integrated circuit implementation. The differences mayvary for changes in temperature of the integrated circuitry. Also thedifferences may vary based on operating mode or band. Rather thanattempt to identify and compensate for each possible variation incomponents along paths 402 and 404, the different illustrativeembodiments of the present disclosure recognize that the distortioncaused by I/Q imbalance results in amount of noise present on digitalsignal 428. For example, distortion caused by the by I/Q imbalance isreflected onto digital signal 428 output by summation unit 426.

In this illustrative embodiment, compensation unit 430 compensates forthe I/Q imbalance between paths 402 and 404. Conjugate unit 432transposes the complex conjugate of digital signal 428 about an axis,for example, the intermediate frequency of receiver 400. Due to theknowledge that the distortion caused by the by I/Q imbalance isreflected onto digital signal 428, the resulting transposed signal isproportional to the distortion present digital signal 428.

The transposed signal output from conjugate unit 432 is weighted byweighting module 434. Weighting module 434 applies weighting factor(s)to the transposed signal to form a weighted signal. Weighting module 434scales the transposed signal to a similar magnitude of the distortionpresent digital signal 428. For example, weighting module 434 may be anarray of programmable digital filters to adapt the transposed signal.

In one example, weighting module 434 includes a finite impulse response(FIR) filter including one or more taps. The number of taps in the FIRfilter may be selected based on the bandwidth of the signal received atreceiver 400. For example, controller 438 may identify the bandwidth ofthe signal and power on or off a number of taps based on the bandwidth.For example, a narrower bandwidth signal, e.g., a global system formobile communications (GSM) signal, may only need one tap, while a widerbandwidth signal, e.g., a long term evolution (LTE) signal may use alarger number of taps, e.g., five taps. Each tap in the FIR filterapplies a weighting factor to a range of frequencies in the transposedsignal. In other words, each tap in the FIR filter scales a portion ofthe total transposed signal. Thus, compensation unit 430 can account forfrequency dependent I/Q imbalances as well as modify the number of tapsfor wideband signals.

Compensation unit 430 subtracts the weighted signal from digital signal428 to reduce the amount of I/Q imbalance distortion present in digitalsignal 428. Decoder 436 then decodes the resulting balanced signal.

In this example, compensation unit 430 additionally includes controller438. Controller 438 includes software and/or hardware for controllingweighting module 434 to output the weighted signal. For example,controller 438 may identify information about operating band(s) oroperating mode (e.g., GSM, CDMA, LTE, LTE advanced, etc.) to control oneor more weighting factors or filter coefficients of weighting module434. In another example, controller 438 may process one or morealgorithms based on inputs from digital signal 428 to control one ormore parameters of weighting module 434.

FIG. 5 illustrates a quadrature receiver with I/Q imbalance compensationincluding an adaptation module according to one embodiment of thepresent disclosure. Receiver 500 is an example of one implementation ofreceiver 400 in FIG. 4. In this illustrative embodiment, receiver 500includes adaptation module 502 to adapt the signal subtracted fromdigital signal 428 based on signal 504 output from compensation unit506. The continuous adaptation allows compensation unit 506 to improvethe I/Q imbalance compensation in real time while the signal is receivedand processed.

As illustrated, adaptation module 502 includes filter bank 508. Filterbank 508 is one or more filters that are used to identify complexcorrelation statistics from signal 504. For example, filter bank 508 mayincludes individual filters for ranges of frequencies in signal 504.

FIG. 6 illustrates an example of one implementation of filter bank 508for a quadrature receiver according to an illustrative embodiment of thepresent disclosure. As illustrated, filter bank 600 includes ‘n’ numberof filters 602 tuned to filter narrower bands of frequencies fromwideband input signal 604. In this example, wideband input signal 604may be signal 504 output from compensation unit 506, digital signal 428output from summation unit 426, or the signal received at receiver 400.Filters 602 can be digitally programmable filters. The number of filters602 can be selected based on a bandwidth of wideband input signal 604.For example, wider bandwidth signals may result in a larger number offilters in filters 602.

Returning to FIG. 5, adaptation module 502 identifies the complexcorrelation statistics from signal 504 for each narrowband signal outputfrom filters 602. Adaptation module 502 adapts coefficients of a FIRfilter in weighting module 434 using a least means squares type ofadaptation algorithm. The adaptation algorithm can be trained on thesignal itself, which enables continuous adaptation in receiver 500. Forexample, the adaptation algorithm may be calculated according toequation 1 below:

w ^((k)) =w ^((k-1)) μ·z(t)[z(t),z(t−T _(s)), . . . z(t−nT_(s))]^(T)  Equation 1.

where w is a vector of the weighting factor for weighting factor update‘k,’ μ is a step size parameter, z(t) is signal 504 output fromcompensation unit 506, n is a tap in the multi-tap filter from 0 to n(e.g., n+1 taps in the multi-tap filter), and T_(s) is the time step ofsignal 504.

FIG. 7 illustrates a quadrature receiver with I/Q imbalance compensationincluding an adaptation module according to another embodiment of thepresent disclosure. Receiver 700 is an example of one implementation ofreceiver 400 in FIG. 4. In this illustrative embodiment, receiver 700includes adaptation module 702 to adapt the signal subtracted fromdigital signal 428 based on digital signal 428 output from summationunit 426. In this illustrative example, the filter bank is implementedusing a fast FFT module 706 to compute distortion parameters of digitalsignal 428 in the frequency domain. For example, the distortionparameters may be computed according to Equations 2 and 3 below:

$\begin{matrix}{g = {\frac{\sqrt{E( {{FFT}_{I}}^{2} )}}{\sqrt{E( {{FFT}_{Q}}^{2} )}}.}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

where g is a distortion parameter for amplitude imbalance, FFT_(I) isthe FFT of the I signal, where FFT_(Q) is the FFT of the Q signal, andE(x) is a function for the expected value of x; and

$\begin{matrix}{\phi = {\frac{\sum{{FFT}_{I}*{FFT}_{Q}}}{\sum{{{FFT}_{I}}{{FFT}_{Q\;}}}}.}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

where φ is a distortion parameter for phase imbalance.

Adaptation module 702 then converts the distortion parameters of digitalsignal 428 in the frequency domain into filter coefficients in the timedomain for the FIR filter in weighting module 434 using an IFFT module708 to compute the filter coefficients. The continuous adaptation allowscompensation unit 704 to improve the I/Q imbalance compensation in realtime while the signal is received and processed. For example, the filtercoefficients of digital signal 428 in the time domain may be calculatedaccording to Equation 4 below:

$\begin{matrix}{w = {{{IFFT}\lbrack \frac{{g\; ^{{- j}\; \phi}} - 1}{{conj}( {{g\; ^{j\; \phi}} + 1} )} \rbrack}.}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

where w is a vector of the weighting factor, and conj(x) is a functionfor the complex conjugate of x.

FIG. 8 illustrates a process for reducing distortion in a processedsignal according to an illustrative embodiment of the presentdisclosure. For example, the process may be implemented by receiver 400in FIG. 4.

The process begins by receiving an analog signal (block 802). Theprocess then processes the analog signal into a digital signal (block804). In block 804, the analog signal is processed by separate I and Qsignal paths. The I and Q signals are summed to form the processeddigital signal.

Thereafter, the process identifies a conjugate of the processed digitalsignal (block 806). In block 806, the complex conjugate of the processeddigital signal may be transposed about an intermediate frequency.

The process then applies a weighting factor to the conjugate of theprocessed digital signal (block 808). In block 808, the process mayidentify a set of weighting factors using frequency independentdistortion identified for one or more ranges of frequencies in theprocessed digital signal using a filter bank including one or morefilters for each frequency range. The set of weighting factors can beapplied to different ranges of frequencies in the conjugate of theprocessed digital signal using a FIR filter having a number of tapsbased on the number of ranges of frequencies. For example, the processmay identify from complex correlation statistics from an output acompensation unit and adapt coefficients of the FIR filter using a leastmean squares algorithm to identify the set of weighting factors.

In another example, the process may identify distortion parameters inthe processed digital signal in a frequency domain and converting thedistortion parameters into coefficients of the FIR filter a time domainto identify the set of weighting factors. The process may also selectthe number of taps in the FIR filter and the number of filters in thefilter bank based on bandwidth of the received analog filter or areceiving mode of the receiver.

Thereafter, the process subtracts the weighted signal from the processeddigital signal (block 810). In block 810, the distortion caused by theI/Q imbalance is reduced by using the weighted signal to approximate thedistortion in the process signal caused by the I/Q imbalance. Thisdistortion is reduced by subtracting out the weighted signal. Theprocess then returns to block 806 and continues to adapt the weightedsignal based on the processed digital signal. The process may terminateupon lack of a signal received at step 802. The process may alsoterminate upon the receiver being powered off.

Thus the different illustrative embodiments provide digital programmablesystems and methods for adaptively compensating and correcting for I/Qimbalance in a quadrature receiver without modifying the analog portionof the receiver. The digital I/Q imbalance compensation system trainsusing received incoming transmitted data signals from a programmablefilter bank to obtain amplitude imbalance and phase imbalanceinformation on which to adaptively converge to a set of I/Q imbalancecompensation coefficients. Depending on the received signal bandwidth,the I/Q imbalance compensation unit can compensate for narrowband,frequency independent IQ imbalance, and for wideband frequency dependentIQ imbalance.

Considering multi-mode multi-band communications network, the presentdisclosure reduces the I/Q imbalance on the digital side, which greatlyreduces the burden in the analog side. Implementation of I/Q imbalancecompensation on the digital side will take advantage of digital circuitprocess scaling. For example, digital I/Q imbalance compensation canimprove signal error rate, and reduce power, size and component costs insubscriber stations and base stations.

One skilled in the art understands various I/Q imbalance contributorscome from the analog and mixed signal blocks within the receiver. Theproposed technique does not rectify these contributors locally, andtherefore, is agnostic to the extent and cause of I/Q imbalance.Further, because the proposed technique does not locally correct thecause of I/Q imbalance, the proposed technique achieves I/Q imbalancecompensation without perturbation to receiver performance or impositionof timing requirements.

Although the present disclosure has been described with an exemplaryembodiment, various changes and modifications may be suggested to oneskilled in the art. It is intended that the present disclosure encompasssuch changes and modifications as fall within the scope of the appendedclaims.

1. For use in a wireless communications network, an apparatus configuredto reduce distortion in a processed signal, the apparatus comprising: afirst receive path configured to process a received analog signal into afirst digital signal; a second receive path configured to process thereceived analog signal with a phase shift into a second digital signal;a summation unit configured to sum the first and second digital signalsto form a processed digital signal; and a compensation unit configuredto identify a conjugate of the processed digital signal, apply aweighting factor to the conjugate of the processed digital signal toform a weighted signal, and subtract the weighted signal from theprocessed digital signal to reduce the distortion.
 2. The apparatus ofclaim 1, wherein the compensation unit comprises: a finite impulseresponse (FIR) filter including one or more taps based on a receivingmode, the FIR filter configured to apply a set of weighting factors tothe conjugate of the processed digital signal to form the weightedsignal.
 3. The apparatus of claim 2, wherein the compensation unitcomprises: a filter bank including one or more filters, the one or morefilters each configured to identify frequency independent distortion forone or more ranges of frequencies in the processed digital signal. 4.The apparatus of claim 3, wherein the compensation unit furthercomprises: an adaptation module configured to identify the set ofweighting factors for the one or more ranges of frequencies in theconjugate of the processed digital signal based on the frequencyindependent distortion identified to form a frequency dependent weightedsignal as the weighted signal to be subtracted from the processeddigital signal.
 5. The apparatus of claim 3, wherein the compensationunit further comprises: an adaptation module configured to identifycomplex correlation statistics from an output of the compensation unitand adapt coefficients for the FIR filter using an adaptation algorithm.6. The apparatus of claim 3, wherein the filter bank comprises: a fastFourier transform (FFT) module configured to identify distortionparameters in the processed digital signal in a frequency domain; and aninverse FFT module configured to convert the distortion parameters intocoefficients for the FIR filter a time domain.
 7. The apparatus of claim6, wherein the compensation unit further comprises: an adaptation moduleconfigured to apply the weighting factor to the conjugate of theprocessed digital signal based on the distortion parameters in the timedomain
 8. The apparatus of claim 2, wherein the compensation unit isfurther configured to identify the receiving mode and select a number oftaps to be included in the FIR filter based on a bandwidth of thereceived analog signal.
 9. The apparatus of claim 1, wherein thedistortion in the processed signal is I/Q imbalance distortion caused bycomponents in the first and second receive paths, and wherein thecompensation unit compensates for the I/Q imbalance distortion in theprocessed digital signal without separate inputs from the components inthe first and second receive paths.
 10. The apparatus of claim 1,wherein the first and second receive paths, the summation unit, and thecompensation unit are part of a quadrature receiver in a subscriberstation in the wireless communications network.
 11. The apparatus ofclaim 1, wherein the first and second receive paths, the summation unit,and the compensation unit are part of a quadrature receiver in a basestation in the wireless communications network.
 12. For use in awireless communications network, a receiver configured to reducedistortion in a processed signal, the receiver comprising: a pair ofquadrature receiver paths configured to process a received analog signalinto a digital signal; a conjugate unit configured to identify aconjugate of the processed digital signal; an adaptation moduleincluding a filter bank, the adaptation module configured to identify aset of weighting factors for one or more frequencies of the conjugate ofthe processed digital signal based on frequency dependent distortionidentified using the filter bank; a weighting module configured to applythe set of weighting factors to the conjugate of the processed digitalsignal to from a weighted signal and subtract the weighted signal fromthe processed digital signal to reduce the distortion; and a decoderconfigured to decode the processed digital signal.
 13. The receiver ofclaim 12, wherein the filter bank includes one or more filters, the oneor more filters each configured to identify frequency independentdistortion for one or more ranges of frequencies in the processeddigital signal.
 14. The receiver of claim 12, wherein the weightingmodule includes a finite impulse response (FIR) filter including one ormore taps based on a receiving mode of the receiver, and wherein theadaptation module is further configured to identify complex correlationstatistics from an input into the decoder and adapt coefficients of theFIR filter using an adaptation algorithm.
 15. The receiver of claim 12,wherein the filter bank includes a fast Fourier transform (FFT) moduleconfigured to identify distortion parameters in the processed digitalsignal in a frequency domain and an inverse FFT module configured toconvert the distortion parameters into coefficients of the weightingmodule in a time domain, and wherein the adaptation module is furtherconfigured to identify the set of weighting factors to the conjugate ofthe processed digital signal based on the distortion parameters in thetime domain.
 16. The receiver of claim 12, wherein the adaptation moduleis further configured to identify a receiving mode of the receiver andselect a number of taps to be included in the FIR filter based on abandwidth of the received analog signal.
 17. A method for reducingdistortion in a processed signal in a receiver in a wirelesscommunications network, the method comprising: receiving an analogsignal; processing the analog signal into a digital signal using a pairof quadrature receiver paths to form a processed digital signal;identifying a conjugate of the processed digital signal; applying aweighting factor to the conjugate of the processed digital signal toform a weighted signal; and subtracting the weighted signal from theprocessed digital signal to reduce the distortion.
 18. The method ofclaim 17, wherein applying the weighting factor to the conjugate of theprocessed digital signal comprises: identifying frequency independentdistortion for one or more ranges of frequencies in the processeddigital signal using filter bank including one or more filters based ona receiving mode of the receiver; and applying a set of weightingfactors to the one or more ranges of frequencies of the conjugate of theprocessed digital signal based on the frequency independent distortionidentified to form a frequency dependent weighted signal as the weightedsignal to be subtracted from the processed digital signal.
 19. Themethod of claim 17, wherein the weighting factor is applied using afinite impulse response (FIR) filter including one or more taps based onthe receiving mode of the receiver, the method further comprising:identifying complex correlation statistics from an output comprising theweighted signal subtracted from the processed digital signal; andadapting coefficients of the FIR filter using an adaptation algorithm.20. The method of claim 17 further comprising: identifying distortionparameters in the processed digital signal in a frequency domain; andconverting the distortion parameters into filter coefficients in a timedomain, wherein applying the weighting factor to the conjugate of theprocessed digital signal comprises: applying the weighting factor to theconjugate of the processed digital signal based on the filtercoefficients in the time domain.